The present invention relates to a pulse width control circuit for controlling an output pulse width in response to a periodic input signal by means of a feedback loop, and more particularly to a pulse width control circuit available in a solid state ignition system for an internal combustion engine of an automobile.
In an ignition system for an internal combustion engine, it is necessary to supply sufficient ignition energy to an ignition coil over the entire speed range of the internal combustion engine (i.e. from a low rotation speed to a high rotation speed). Both the timing and duration of ignition energy supplied to the ignition coil are controlled by making use of a signal synchronized with the rotation of the internal combustion engine. Specifically upon the low engine speed rotation, cycle period of the synchronized signal is long; while upon high speed operation the cycle period of the synchronized signal is short. In addition, the ignition coil is an inductive load such that a current flows through the ignition coil with a time constant determined by the inductance and resistance of the ignition coil. In view of the aforementioned operating conditions, it is thus necessary to select the period for supplying a current to the ignition coil to be short with respect to a cycle period of the input signal upon low speed rotation, and to be long with respect to a cycle period of the input signal upon high speed rotation. The absolute value of the current suppling period to the ignition coil is nearly equal upon the low speed operation and upon the high speed operation.
For this purpose, the ignition system for an internal combustion engine typically includes a pulse width control circuit for controlling the pulse width of an output pulse in response to an input signal by means of a feedback loop. The pulse width control circuit in the ignition system produces a pulse drive signal for supplying a current to an ignition coil by making use of both an input signal synchronized with the rotation of the internal combustion engine and a comparison output signal obtained by comparing a sawtooth signal derived from the input signal with a reference level. The pulse drive signal consists of a current supply start point and a current supply period. A current flowing through the ignition coil is detected, and the detected output is fed back to change the reference level. In other words, a feedback loop is provided. By this feedback loop, the abovementioned reference level is varied depending upon the rotational speed of internal combustion engine, and a pulse drive signal corresponding to the rotational speed is thereby obtained.
However, since the pulse drive signal is produced by making use of the sawtooth signal derived from the input signal as described above, the amplitude level of the sawtooth signal influences the pulse width and the start point of the pulse drive signal. The amplitude level of the sawtooth signal is determined by a duty ratio of the input signal, and the duty ratio of the input signal is in turn determined by the rotation mode of the internal combustion engine. Where the internal combustion engine is started when the outside temperature is low as in a severe winter, smooth rotation of the engine cannot be obtained due to the high viscosity of engine oil. For this reason, the duty ratio of the input signal under these conditions varies, and hence the amplitude level of the sawtooth signal will become too large. Consequently, it often occurs that the comparison output cannot be derived, and the pulse drive signal for supplying a current to the ignition coil thus cannot be produced.
Hence, it has been proposed to vary the reference level so that the comparison output may be obtained even when the sawtooth signal has an abnormal amplitude level. However, when the viscosity of the engine oil decreases due to the temperature increase caused by the continuous operation of the engine, or when the outside temperature rises, an input signal having the predetermined duty ratio will be obtained. Further, the reference level will remain at the level established in response to the sawtooth signal having the abnormal amplitude level. Consequently, the pulse width of the pulse drive signal for the ignition coil will become too large, and the current supply period to the ignition coil will become unnecessarily long. Since the ignition system is additionally provided with a current limiting capability for limiting a peak value of the supply current to the ignition coil, the breakdowns of circuit elements such as transistors due to an excessively large current will not typically occur. However, in some cases, thermal breakdowns of the driving transistors caused by heat generation therein may possibly occur due to this overly long current supply period. When it is intended to prevent the thermal breakdown in the solid state ignition system by relying upon a heat capacity and a heat dissipation characteristic of an equipped capsule, the thermal design of semiconductor devices such as transistors which operate below their capable junction temperatures would be accompanied by a great deal of difficulty. Moreover, taking into consideration the great range of operating temperatures of the internal combustion engine, this design difficulty is increased even further. In addition, as a matter of course, the electric power consumption is also very high.